1. Field of the Invention
This invention relates to image processing.
2. Description of the Prior Art
An example problem will be described in the field of face detection, but the invention is generally applicable to detection of different types of objects such as faces, cars, animals and the like, and to image processing in general.
Various object detection techniques, for example for human face detection, have been proposed. It is generally possible to detect a human face with a reasonably high degree of certainty in a captured image (e.g. a frame of a video signal).
Moving further, it is desirable to be able to associate together detected faces in different images, so as to generate data representing, for example, how long a single face stayed in view of a camera (a so-called dwell time). This is of use in retail applications (for example, to detect how long a customer browsed a particular shelf in a store) or security applications. Techniques for achieving this are described in WO2004/051553 and generally involve matching face positions and face properties between temporally adjacent images, with an allowance for reasonable inter-image movement.
Going further still, it would be desirable to be able to link together face tracks obtained at different times and/or from different cameras. In order to do this, it is often necessary to compare image properties (e.g. color properties) of image areas between the different tracks, in order to arrive at a decision as to whether the tracks relate to the same person or different people.
However, because the tracks could well have been generated by different cameras, or under different lighting conditions, even where the person is indeed the same between tracks, the results of such a comparison can indicate otherwise.